Formation and dissociation kinetics of the complexes Gd(DOTP)(5-) and Gd(DOTPMB)(-)

The monobutyl ester of H8DOTP, the Ligand H4DOTPMB, was synthesized. and th e protonation constants (K-i(H)) and the stability constant of Gd(DOTPMB) w ere determined by pH-potentiometry (25 degreesC. 0.1 M Me4NCl): logK(i)(H) (i= 1, 2, and 3) = 10.34(0.02), 7.72(0.025), and 2.42(0.030), respectively. and logK(GdL) = 12.19(0.05). The rates of formation of Gd(DOTPMB) and Gd(D OTP) were studied by H-1 relaxometry in the pH range 5.4-7, and also by spe ctrophotometry in the case of Gd(DOTP) (7 < pH < 8). For both reactions, fi rst-order rate constants were obtained at different concentration ratios of the reactants, which indicated the rapid formation of a reaction intermedi ate. The compositions of the intermediates are Gd(HiDOTPMB) (i = 1, 2) and HnGd(H2DOTP) (n = 0-4), respectively, where one or two protons are attached to the nitrogen atoms of the ligand. The rate of rearrangement (k(r)) of t he intermediate Gd(HiDOTPMB) to the product Gd(DOTPMB) increases with incre asing [OH-]: k(r) = k(OH)[OH-] + k(2OH)[OH-](2), where k(OH) = (1.3+/-0.25) x10(3) M(-1)s(-1) and k(2OH) = (7.8+/-0.23 x10(11) M(-2)s(-1) For the form ation reaction of Gd(DOTP), only the first term exists and k(OH) = (7.2+/-0 .1) x10(3) M(-1)s(-1). For the complexation reactions, similar mechanisms w ere proposed in which deprotonation of the species Gd(HDOTPMB) and HnGd(HDO TP) plays the rate-determining role. For the deprotonation, general base ca talysis was found to be satisfactory. The rate of dissociation of Gd(DOTPMB ) in 0.025-1.0 M HCl solution ([HCl] + [Me4NCl] = 1.0 M) was lower than tha t of Gd(DOTP) and the first-order rate constants exhibited saturation curve s with increasing [HC]. Based on the assumption that the protonated species HGd(DOTPMB) and H5Gd(DOTP) dissociate, the rate constants land protonation constants) were found to be (5.4+/-0.2) x10(-4) M(-1)s(-1) (K-GdL(H) = 1.7 +/-0.1) and (2.1+/-0.1) x10(-4) M(-1)s(-1) (K-H4GdL(H) = 1.9+/-0.1), respec tively.